1. Field of the Invention
The present invention relates to a miniaturized optical head provided for equipping the distal end of a flexible optical fibre bundle, said head being intended to be placed in contact with an analyzing surface and adapted for focussing an excitation signal conveyed by said fibre bundle into an excitation focal point which can be situated at different depths relative to the contact surface of the head. The optical head is also adapted for collecting a backscattered signal originating from the subsurface excitation focal point in order for it to be carried by the fibre bundle in particular to detection means and means of analysis and digital processing of the signal.
The fields of application concerned are subsurface analysis devices which are confocal in character, the signals conveyed being in particular those of the field of imaging and/or spectroscopy, depending on the source or sources of excitation and detection means used. The confocal character results from the use of the same fibre or fibres for conveying the excitation signal and the backscattered signal. The device can be used for in situ biological analyses, on humans or animals, external, for example in the field of dermatology, or internal and accessible using the instrument channel of an endoscope into which the optical fibre bundle and the optical head can be introduced. It can also be used for cell analyses carried out ex vivo on samples. Moreover also, the optical head can be used for analyzing the interior of a manufactured device.
At present, the medical fields of gastroenterology, respirology, gynaecology, urology, otorhinolaryngology, dermatology, ophthalmology, cardiology and neurology are concerned.
The magnification of the optical head according to the present invention may or may not be unitary. It is the analysis and signal processing means provided on the side of the proximal end of the optical fibre bundle which allow the restitution of an image or a graph which can be interpreted by a user.
The sought objectives for the optical head are in particular the following:                to have a minimal space requirement in particular in order to be able to be inserted into the instrument channel of an endoscope which in general possesses a diameter comprised between 2 and 4 mm and a given radius of curvature.        to provide a good quality backscattered signal in which aberrations are minimized;        to minimize parasitic reflections at the distal output of the fibre bundle;        to provide a spatial resolution of the excitation focal point of the order of 4 μm, or even less in the case of a non-unitary magnification, allowing the analysis and/or observation of a tissue at a cellular scale;        to be able to be brought into contact with the analyzing surface in order to avoid the problems linked with untimely movements; and        to allow a point focussed in a section plane XY situated at a given depth from the analyzing surface.        
Miniaturization of the optical head is also advantageous in order to increase the precision of its positioning and also to minimize mechanical inertia in automated uses, for example in extension of a robot arm or telemanipulator.
2. Description of the Related Art
From the document WO 00/16151, an observation device is known comprising an optical focussing head at the distal end of a flexible channel of optical fibres comprising at the channel output successively three lenses: a ×10 microscope objective, a doublet of 150 mm focal length and a doublet of 50 mm focal length.
An optical head is also known, comprising a system of four lenses, the first lens and the fourth lens being two ×10 microscope objectives and the second and third lenses two doublets of 150 mm focal length constituting an afocal system of magnification 1.
These optical systems have the following major drawbacks:                this type of construction, based on sophisticated microscope objectives (which can contain up to twelve lenses) cannot be miniaturized in order to be introduced into an endoscope instrument channel with a diameter of 2 to 4 mm;        the lateral resolution is of the order of 8 μm, insufficient for analyzing a tissue on the cellular scale;        in the case of confocal imaging, with illumination and scanning of the fibres one by one, a distortion of the image formed is observed (“ballooning” of the lines).        